1. Field of the Invention.
The present invention relates to the field of data storage devices.
2. Prior Art.
High speed data processing systems increasingly require associated storage devices capable of storing massive quantities of data. Historically, large quantities of data have been stored in magnetic media such as tapes, drums and disks. All such storage media require mechanical movement of a magnetically coated storage surface past one or more "read" heads. Memory access time is thus limited by the cycling time of the mechanical drive.
Semiconductor memories afford rapid data access, however, even the most efficient of such memories dissipate too much energy to make truly massive storage capacities practical.
It has long been theorized that vast quantities of data could be stored optically and that such data could be retrieved literally at the speed of light. Although optical devices for storing limited quantities of data pulses have been demonstrated, practical devices for dynamic optical storage of very large quantities of data have heretofore not been achieved.
Research in the field of optical data storage has been reported from the Soviet Union. For example, M. I. Belovolov et al have disclosed a recirculating memory in an article entitled "Dynamic Direct-Access Memory Utilizing Fiber Waveguides" appearing in Kvantovaya Elektron (Moscow) 2, 214-216 (Jan. 1985). An English translation of this article appeared in Sov. J. Quantum Electronics 15(1), Jan. 1985, pp. 137-139. Belovolov et al propose a dynamic direct access memory having a graded-index fiber waveguide 5 km in length. A laser diode emitter is encoded with digital data. The resulting sequence of light pulses is transmitted through the fiber waveguide and detected at the exit end by a photodiode. The detected signal is synchronized with the data clock and reapplied to the laser diode. Using TTL circuitry, Belovolov et al achieved a storage capacity of 103 bits at a clock frequency of approximately 10 MHZ. Channel multiplexing was suggested as one means of increasing memory capacity without increasing waveguide length. However, data rates for any one channel are inherently limited (2 GHZ is suggested as achievable) by the electrical regeneration techniques used.
Further work on the concepts of Belovolov et al is reported in an article by Yu. V. Gulyaev et al entitled "Fiber-Optics Element for Information Storage" appearing in Pis'ma Zh. Tekh. Fiz. 12, 350-354 (Mar. 26, 1986). An English translation of this article appeared in Sov. Tech. Phys. Lett. 12 (3), Mar. 1986, pp 143-145. Gulyaev et al describe a dynamic memory consisting of an optical fiber connected in a closed loop by an optical pulse regenerator. Digital information is encoded as bursts of emission of a semiconductor laser. The light pulses propogate through a single mode optical fiber and are reflected by a mirror coating at the exit end. Regeneration is achieved in the laser by generation of pump current pulses when the reflected optical pulses are injected back into the active region of the laser. The apparatus of Gulyaev et al thus reshapes the coded waveform on each circulation through the optical fiber storage medium without incurring the delay of electrically decoding the optical signal. However, since regeneration is accomplished within the laser itself, it is not possible to multiplex a plurality of storage channels within the bandwidth of the laser. Therefore, overall data density can be increased only by increasing the length of the fiber or by multiplexing storage channels having dedicated laster emitters.
A frequency multiplexed fiber optic data storage system is described by d'Auria et al in U.S. Pat. No. 4,653,042 issued Mar. 24, 1987 and entitled "Device for Storing Information in an Optical Fiber Transmission System." d'Auria et al disclose a method for using an existing fiber optic data transmission line as a storage medium. Data is transmitted from one station to another at a first wave length. Data to be stored is encoded on an emitter operating at a second wavelength and injected into the fiber optic line. A dichroic mirror at a remote station passes light at the data transmission wave length but reflects light at the data storage wavelength. Stored data is reflected back to the originating station where it is detected and converted to an electrical signal which is then reapplied to the data storage emitter to regenerate the stored data as an optical signal.
As in other systems employing opto-electronic signal regeneration, the storage capacity of d'Auria et al's apparatus is inherently limited by the operating frequency of the opto-electronic devices. 100 MHZ is suggested as a realistic operating frequency, thereby resulting in a storage capacity of approximately 103 bits per km per channel.
An optical storage device that does not depend on optoelectronic regeneration is described by Shaw in U.S. Pat. No. 4,473,270 issued Sept. 25, 1984 and entitled "Splice-Free Fiber Optic Recirculating Memory." Shaw discloses a fiber optic memory in which a single mode optical fiber is coupled to itself to form a closed-loop which acts as a delay line. Coupling is achieved by juxtaposition of polished tangential surfaces at each end of the fiber so that a portion of the light which is leaving the loop is coupled back into the loop. When data is injected into the loop, the output will be a serial repetition of the input signal with decreasing amplitude at intervals corresponding to the length of the loop. Because only a portion of the light signal is coupled back into the loop without amplification, the data signal quickly decays so that long term storage of data is impractical.
It is apparent that none of the prior art optical storage devices provide a practical means for storing massive quantities of data for extended periods of time. All such systems either require opto-electronic signal regeneration, thereby limiting the data rate; or employ optical regeneration techniques that limit the data bandwidth and/or the storage duration Furthermore, prior art systems that have employed frequency multiplexing to increase storage capacity have required a dedicated laser emitter for each storage channel, thereby imposing practical limits on the extent of parallelism that can be achieved.
With respect to the present invention the general construction, operation and technology of the various high speed optical devices used therein are described in various articles including the following; "Multiple Quantum Wells for Optical Logic", Arthur L. Robinson, Science Vol. 225, Aug. 1984 ; "Optical Computing", Jeff Hecht, Computer & Electronics Jan. 1985; "Materials for Optical Information Processing", A. M. Glass, Science Vol. 226, Nov. 1984.